I'm not a passionate gamer: I like very much games, but
expecially from a technical/developer point of view: often they are the
expression of the state-of-the-art of many fields, condensated in one
product. Think about algorithms, graphics, artificial intelligence,
physical simulation and numerical computation. I love games, I
like very much experimenting with graphics, but I am a sporadic gamer
(with the exception of RPGs like Star Wars KOTOR I and II, NeverWinter
Nights).

Back at my third year at the university, however, some of my fellow studens formed an Enemy Territory clan. Enemy Territory
is a multiplayer only, team based FPS on the second world war. It is
free, since it should have been developed as the multiplayer part of a
single player game, but the project for the latter failed.
You can chose your team, axis or allies, and a class: soldier,
medic, engineer, covert ops, field ops. Every class has unique
strengths and abilities: it's very fun to be a medic and "revive" your
friends, or to be a covert op and steal the uniform of a member of the
other team, or to be a sniper....

The absence of single-player capabilities, and of an artificial
intelligence at all (no bots) it's stimulating: every opposer is a
human, and so there is no predefined tactis or "stupid" behaviours
(well, only humanly stupid behaviour).

There are a stock of predefined sentences to communicate quickly with
other member of the team over the internet, when you can reach the
other "by voice", like "I need a medic", "They are coming", etc. Among
them, "We need an engineer" (mainly to defuse dynamite) and "Enemy in
disguise" (when you see a covert ops with your uniform..). The
sentences came as both a written banner at the bottom of the screen and
an audio file. In the shipped version the written sentences are equal
to the audio files, but it is possible to change the written part.

Someone on the internet made a thing that made me laugh for an hour or
so: they changed "We need an engineer" with "We need a ninja here" and
"Enemy in disguise" with "Enemy in the skies". Same pronunciation! And
I saw someone from my team looking up watching the skies..And when things get really bad, t's so cool to go around and call a ninja to help you =)